1. Field of the Invention
Superalloys with a nickel base which, owing to their outstanding mechanical properties at high temperatures, are used in the construction of heat engines subjected to high thermal and mechanical load. Preferred use as bucket material for gas turbines.
The invention relates to the further development of nickel-base superalloys with emphasis on cast alloys for directional solidification.
In particular, it relates to a precipitation-hardenable nickel-base superalloy with improved mechanical properties in the temperature range from 600.degree. to 750.degree. C.
It further relates to a process for manufacturing a structural component from the precipitation-hardenably nickel-base superalloy by melting and casting the alloy, its crystallites being forced to solidify in a directional manner and then subjecting it to a heat treatment.
2. Discussion of Background
The following literature is cited in relation to the prior art:
Robert W. Fawley, Superalloy progress, The Superalloys p. 3-29, edited by Chester T. Sims and William C. Hagel, John Wiley and Sons, New York 1972;
Michio Yamazaki, Development of Nickel-base Superalloys for National Project in Japan, High temperature alloys for gas turbines and other applications, 1986, pages 945-953, Proceedings of a conference held in Liege, Belgium, 6-9 October 1986, D. Reidel publishing company, Dordrecht.
Of the commercially available nickel-base cast alloys, the alloy having the trade name IN 738 manufactured by INCO is often used. It has the following composition:
Cr=16 percent by weight PA1 Co=8.5 percent by weight PA1 W=2.6 percent by weight PA1 Mo=1.75 percent by weight PA1 Ta=1.75 percent by weight PA1 Al=3.4 percent by weight PA1 Ti=3.4 percent by weight PA1 Zr=0.1 percent by weight PA1 B=0.01 percent by weight PA1 C=0.11 percent by weight PA1 Ni=remainder PA1 Cr=12.4 percent by weight PA1 Co=9 percent by weight PA1 W=3.8 percent by weight PA1 Mo=1.9 percent by weight PA1 Ta=3.9 percent by weight PA1 Al=3.1 percent by weight PA1 Ti=4.5 percent by weight PA1 Zr=0.1 percent by weight PA1 B=0.02 percent by weight PA1 C=0.12 percent by weight PA1 Ni=remainder PA1 Cr=12-15 percent by weight PA1 Co=3-4.5 percent by weight PA1 W=1-3.5 percent by weight PA1 Ta=4-5.5 percent by weight PA1 Al=3-4.3 percent by weight PA1 Ti=4-5 percent by weight PA1 Hf=0-2.5 percent by weight PA1 B=0-0.02 percent by weight PA1 Zr=0.01-0.06 percent by weight PA1 C=0.05-0.07 percent by weight PA1 Ni=remainder PA1 (a) heating to 1,100.degree. C. under argon atmosphere, PA1 (b) keeping at 1,100.degree. C. for 10 h, PA1 (c) heating to 1,220.degree. C. at a rate of 30.degree. C./h, PA1 (d) keeping at 1,220.degree. C. for 2 h under argon atmosphere, PA1 (e) heating to 1,270.degree. to 1,280.degree. C. at a rate of 30.degree. C./h under argon atmosphere, PA1 (f) keeping at 1,280.degree. C. for 10 h under argon atmosphere, PA1 (g) cooling to room temperature at a rate of at least 10.degree. C./min, PA1 (h) heating to 850.degree. C., PA1 (i) keeping at 850.degree. C. for 4 h in air, PA1 (k) cooling to room temperature at a rate of at least 10.degree. C./min, PA1 (1) heating to 760.degree. C., PA1 (m) keeping at 760.degree. C. for 16 h in air, PA1 (n) cooling to room temperature at a rate of at least 10.degree. C./min.
In many cases this alloy does not satisfy the long-term requirements imposed on industrial gas turbines in relation to creep resistance. In addition, it contains not insignificant quantities of the expensive strategic metal cobalt.
The alloy having the trade name IN 792 manufactured by INCO should be mentioned as a further commercial nickel-base cast superalloy which is used in gas turbine construction. It has the following composition:
This alloy is also unsatisfactory in relation to its creep behavior during long-term loading. In addition, its corrosion resistance tends to be at the lower limit in the temperature range of interest.
There is therefore a need to improve the existing alloys in particular in relation to their long-term use.